Selective recovery of iron and aluminum from red mud wastewater using fluidized bed homogeneous crystallization

Abstract

Red mud is a hazardous byproduct originating from the Bayer process during bauxite ore refining that contains substantial quantities of valuable metal ions including iron and aluminum. However, its high alkalinity and large volume pose considerable environmental concerns. In this present work, we developed a two-stage fluidized bed homogeneous crystallization (FBHC) technology for the selective and sequential recovery of ferric hydroxide (Fe(OH)₃) and bayerite (Al(OH)₃) from real red mud leachate, without the utilization of heterogeneous seed materials. The crystallization of Fe was optimized at pH 4.0 ± 0.2 and hydraulic retention time (HRT) of 30 min, while Al recovery via FBHC was most effective at pH 9.5 ± 0.3 and up-flow velocity (U) = 25.0 m/h. Iron and aluminum were correspondingly recovered as ferric hydroxide (Fe(OH)₃) and bayerite (Al(OH)₃) with total recovery efficiencies reached 99.89 % for Fe(III) and 99.6 % for Al(III). The FBHC technology developed in this study overcomes the limitations of conventional heterogeneous crystallization methods by producing pure products without the need for seed materials and generating less sludge. The resulting crystallized products exhibit uniform spherical shapes (0.5–1.0 nm) and well-defined crystal structures, as confirmed by SEM, EDS, XRD, and FTIR analyses. A preliminary cost-benefit analysis indicated that the proposed process has potential for upscaling and economic implementation in red mud valorization. Recovering metals from red mud helps reduce environmental risks whereas generating economic benefits.